X-ray tube comprising a cast copper anode sealed with a copper-silver electric alloy



Sept. 8, 1959 T. RAlNE 2,903,611

X-RAY TUBE COMPRISING A CAST COPPER ANODE SEALED WIT PER-SILVER ELECTRIC ALLOY ed April 24, 1956 H A COP Fil I INVENTOR ATTORNEYS United States Patent X-RAY TUBE COMPRISING A CAST COPPER AN- ODE SEALED WITH A COPPER-SILVER ELEC- TRIC ALLOY Thomas Raine, Bramhall, England, assignor to Metropolitan-Vickers Electrical Company Limited, London, England, a British company Application April 24, 1956, Serial No. 580,407

Claims priority, application Great Britain May 6, 1955 1 Claim. (Cl. 31355) This invention relates to the treatment of a body of (vacuum) cast copper whereby to close the grain boundaries therein and render the body capable of forming part of a vacuum-tight enclosure. Whilst the invention has particular application to the production of liquid-cooled anodes for X-ray tubes, it is also useful for the treatment of copper members destined to form part of the envelope of an evacuated enclosure.

It is known that when copper is cast under vacuum, minute cracks, which may cause vacuum leaks, develop between the crystals of the metal, and these boundaries are further weakened if the metal is subjected to an oxidising atmosphere when hot and is subsequently treated in a reducing atmosphere. These cracks arise from the oxidation at the boundaries of the crystals and the subsequent reduction of the oxide so formed during the heating in the reducing atmosphere. The treatment may, however, be an essential part of a manufacturing process for producing parts from a block of cast copper. Furthermore, the grain boundaries also constitute weak mechanical links in the structure of the body.

It is the object of the invention to provide a treatment which will avoid the production of such minute cracks and thereby render cast copper vacuum-tight and mechanically strong.

According to the invention, the body of cast copper is coated with a layer of copper-silver (eutectic) alloy, and the coated body is then heated in a reducing atmosphere in order to reduce the oxide layers between the grain boundaries and effect the infiltration of the eutectic alloy into the copper.

The coating of the copper body with the eutectic alloy is preferably effected by spraying, and the heating in a reducing atmosphere may take place at a temperature of about 800 C., hydrogen being the preferred gas.

In the accompanying drawings, there is illustrated, by way of example, an X-ray tube having an anode of cast copper, the grain boundaries in which are sealed by the method of the invention.

In the drawings, Fig. 1 is a view, partly in crosssection, of an X-ray tube showing the anode thereof, and Fig. 2 is a detail view of the anode in cross-section.

Referring to the drawings, the X-ray tube is provided with a metal envelope 1, which may be formed from an oxygen-free solid drawn copper tube. To the end of the envelope is secured by way of a collar 2 and sealing sleeve 3 a glass tubular portion 4 which serves insulatingly to support the electron gun 5, the gun being supported by way of a flexible metal annulus 6. The gun 5 incorporates a directly heated cathode 7 and a surrounding: shield 8 which serves. todirect the electrons emitted from the cathode into a beam which falls upon target disc 8, forming part of the vacuum-cast copper anode 9. The anode 9 is sealed to, and terminates one end of, the envelope tube 1. X-rays emitted from the anode as a result of its bombardment by the cathode beam pass through a window 10 in the tube 1.

Anode 9 is hollow so that it may be fluid cooled and in Fig. 2. a detail thereof is shown to indicate the coatings preferably applied thereto. The eutectic coppersilver alloy covering the external surface of the anode, except for the target disc 8, is indicated at 11.

In carrying out the process of the invention in connection with a hollow copper body 9, the disc or plate 8, usually of tungsten, which serves as a target for the electron beam in the tube, is incorporated into the anode. To this end, the formation of the anode may be effected by casting the copper under vacuum in a mould into which the disc 8 is inserted, and cooling the cast body slowly and progressively from the bottom end of the casting where the disc is situated. This progressive cooling results in a coarse grain size in the copper. In order to seal the boundaries between the grains and prevent them from developing vacuum leaks during the heating and cooling processes involved in the manufacture of an X-ray tube, the interior hollow portion of the cast anode body 9 may be nickel-plated as indicated at 12, by electrolytic deposition to a thickness of between 0.001" and 0.005". The coated body is then heated, as above mentioned, in order to bond the nickel to the copper by inter-diffusion. The casting is then machined to the required size and if necessary, to remove any adherent copper from the face of the disc, and the machined surface is then roughened, by shot-blasting. During this operation, the disc 8 is shielded by a disc of neoprene or similar material. The anode is then boiled in water for 15 minutes to loosen and remove any grit left by the shot-blasting operation.

Finally, it is degreased in a suitable solvent such as trichlorethylene. The surface of the anode is next metal-sprayed from a Shoop or similar spray gun as disclosed for example in Patents Nos. 1,133,507 and 1,133,508, both dated March 30, 1915, with a coppersilver (eutectic) alloy, a layer about 0.005 thick being applied as indicated at 11. This does not adhere to the tungsten target 8, so it is not necessary to shield the tungsten disc in this operation, although if other materials were used, this would probably be necessary. The coated anode is now heated in vacuum or a reducing atmosphere such as hydrogen to 800 C., and held at this temperature for one minute only. This treatment is just sufiicient to cause the copper-silver (eutectic) alloy to melt and wet the surface of the copper and penetrate the grain boundaries between the crystals, the melting point of a copper-silver eutectic alloy being 775 C.

Anodes for X-ray tubes which have been given such treatment have been found to be entirely free from vacuum leaks.

What I claim is:

An X-ray tube comprising an evacuated envelope, an electron emitting cathode, and an anode structure contained within said envelope, said anode structure being constructed to provide for the passage of a cooling fluid therethrough, said anode structure including an anode formed from vacuum-cast copper having a button of refractory metal embedded therein, upon which button the electrons from said cathode impinge during operation of said tube, said anode having a surface coating of copper-silver eutectic alloy formed thereon and bonded thereto and sealing the grain boundaries in said copper.

References Cited in the file of this patent UNITED STATES PATENTS 4 Ehrke Sept. 8, Driggs Nov. 9, Sharp Nov. 9, Trembley Apr. 18, Alexander June 20, Shoupp Sept. 19, Bolton June 10, Vosler Aug. 21, Shipe J an. 22,

OTHER REFERENCES Reinhold Publishing Corp., 330 W. 42nd Street, New York (1940) page 70 relied on. 

